The Geomagnetic Field and its Role in Directional Orientation
293
2 Functional Characteristics of Magnetic Compass
Mechanisms
The functional mode of the magnetic compass has been analyzed in very few
species only, with birds being by far the best-studied group, but this limited data
base already shows that a common mechanism of magnetic compass does not exist
among animals. Even among vertebrates, two types of mechanisms based on
different features of the magnetic field have been described.
2.1 Functional Modes
According to the way in which directional information is derived from the field
lines, two types of mechanisms can be distinguished: an inclination compass and a
polarity compass.
2.1.1 The Inclination Compass
The inclination compass was first described for migratory birds. As already
mentioned, the birds reversed their directional preference when the horizontal
component of the ambient magnetic field was reversed (Fig. 2b ). However, they
reversed their preferences also when the vertical component was inverted (Fig.
2c). When both components were reversed, i.e. when the axis of the field lines
remained constant, just changing its polarity, the test birds responded in the same
way as they did in the geomagnetic field (W. Wiltschko and R. Wiltschko 1972).
Birds obviously ignore the polarity of the magnetic field and derive directional
information from the inclination of the axis of the field lines. This compass is
fundamentally different from our technical compass: instead of magnetic North
and South, it distinguishes poleward, the direction where the axis of the field lines
forms the acute angle with gravity, and equatorward, where this angle is larger
than 90° (Fig. 3). At the magnetic equator, where the field lines run horizontally,
the two angles are equal and the inclination compass provides bimodal information. In cage tests, this leads to disoriented behavior (W. Wiltschko and
R. Wiltschko 1972).
An inclination compass as described above was found in all bird species tested
so far, in passerine migrants as well as in homing pigeons. It might thus be a
common feature within the avian class (seeR. Wiltschko and W. Wiltschko 1995).
The inclination compass is not restricted to birds, however. In the critical test
situation with the vertical component of the ambient field inverted, aquatic
salamanders, Notophthalmus viridescens (Salamandridae), and young loggerhead
sea turtles, Caretta caretta (Cheloniidae), reversed their headings, indicating that
their magnetic compass, too, is an inclination compass (Phillips 1986a, Light et al.
1993, Goff et al. 1998).
293
2 Functional Characteristics of Magnetic Compass
Mechanisms
The functional mode of the magnetic compass has been analyzed in very few
species only, with birds being by far the best-studied group, but this limited data
base already shows that a common mechanism of magnetic compass does not exist
among animals. Even among vertebrates, two types of mechanisms based on
different features of the magnetic field have been described.
2.1 Functional Modes
According to the way in which directional information is derived from the field
lines, two types of mechanisms can be distinguished: an inclination compass and a
polarity compass.
2.1.1 The Inclination Compass
The inclination compass was first described for migratory birds. As already
mentioned, the birds reversed their directional preference when the horizontal
component of the ambient magnetic field was reversed (Fig. 2b ). However, they
reversed their preferences also when the vertical component was inverted (Fig.
2c). When both components were reversed, i.e. when the axis of the field lines
remained constant, just changing its polarity, the test birds responded in the same
way as they did in the geomagnetic field (W. Wiltschko and R. Wiltschko 1972).
Birds obviously ignore the polarity of the magnetic field and derive directional
information from the inclination of the axis of the field lines. This compass is
fundamentally different from our technical compass: instead of magnetic North
and South, it distinguishes poleward, the direction where the axis of the field lines
forms the acute angle with gravity, and equatorward, where this angle is larger
than 90° (Fig. 3). At the magnetic equator, where the field lines run horizontally,
the two angles are equal and the inclination compass provides bimodal information. In cage tests, this leads to disoriented behavior (W. Wiltschko and
R. Wiltschko 1972).
An inclination compass as described above was found in all bird species tested
so far, in passerine migrants as well as in homing pigeons. It might thus be a
common feature within the avian class (seeR. Wiltschko and W. Wiltschko 1995).
The inclination compass is not restricted to birds, however. In the critical test
situation with the vertical component of the ambient field inverted, aquatic
salamanders, Notophthalmus viridescens (Salamandridae), and young loggerhead
sea turtles, Caretta caretta (Cheloniidae), reversed their headings, indicating that
their magnetic compass, too, is an inclination compass (Phillips 1986a, Light et al.
1993, Goff et al. 1998).